[clang-format] a lambda with a qualified trailing return type is split after :: when its body breaks a << chain with comments #241859

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   @llvm/issue-subscribers-clang-format

   Author: Michael Aivazis (aivazis)

   Details

   ## Summary

   clang-format 23.1.1 breaks the line after the :: of a lambda's qualified trailing return type and
   indents the whole body to the column of the unqualified name, when the body contains a << chain
   whose operands are separated by comment lines. The same lambda formats as expected when its return
   type has no scope, or when the chain carries no comments.

   Reproduction

   The .clang-format of the project where it was found is reproduced below; with it,
   the input below reproduces the problem. A configuration reduced to BasedOnStyle: LLVM plus the
   project's column limit, indentation and operator settings does not, so the trigger involves another
   of the project's options, not yet isolated.

   input.cc:

   void
   f(cls_t & cls)
   {
       cls.def("iq", [](const DataSet & self) -> py::object {
           auto channel = make();
           channel
               // where
               << here()
               // flush
               << endl;
           return none();
       });
   }

   Expected

   The input unchanged, apart from the placement of the arguments of def.

   Actual

       cls.def(
           "iq",
           [](const DataSet & self) -> py::
                                        object {
                                            auto channel = make();
                                            ...

   Comments in the body are then wrapped against the column limit from the new, deeper indentation.

   Observations

   • -> py::object, -> pybind11::object and -> ::pybind11::object all trigger it; -> object_t,
     an alias without a scope, does not.
   • Removing the comment lines from the << chain makes it go away; so does keeping the comments and
     using a named function with the same trailing return type and body.

   Version

   clang-format version 23.1.1 (conda-forge), macOS 27, arm64.

   <details><summary>the project's <code>.clang-format</code></summary>

   # -*- yaml -*-
   # -*- coding: utf-8 -*-
   #
   # michael a.g. aïvázis &lt;michael.aivazis@<!-- -->para-sim.com&gt;
   # (c) 1998-2026 all rights reserved
   
   
   ---
   Language:        Cpp
   AccessModifierOffset: -4
   AlignAfterOpenBracket: AlwaysBreak
   AlignConsecutiveAssignments: false
   AlignConsecutiveBitFields: false
   AlignConsecutiveDeclarations: false
   AlignConsecutiveMacros: false
   AlignEscapedNewlines: true
   AlignOperands: AlignAfterOperator
   AlignTrailingComments: true
   AllowAllArgumentsOnNextLine: true
   AllowAllParametersOfDeclarationOnNextLine: true
   AllowShortBlocksOnASingleLine: false
   AllowShortCaseLabelsOnASingleLine: false
   AllowShortEnumsOnASingleLine: true
   AllowShortFunctionsOnASingleLine: Inline
   AllowShortIfStatementsOnASingleLine: Never
   AllowShortLambdasOnASingleLine: Inline
   AllowShortLoopsOnASingleLine: false
   AlwaysBreakBeforeMultilineStrings: true
   BinPackArguments: true
   BinPackParameters: true
   BraceWrapping:
     AfterCaseLabel: false
     AfterClass: false
     AfterControlStatement: Never
     AfterEnum: false
     AfterFunction: true
     AfterNamespace: false
     AfterStruct: false
     AfterUnion: false
     AfterExternBlock: false
     BeforeCatch: false
     BeforeElse: false
     BeforeLambdaBody: false
     BeforeWhile: false
     IndentBraces: false
     SplitEmptyFunction: false
     SplitEmptyRecord: false
     SplitEmptyNamespace: false
   BreakAfterReturnType: TopLevel
   BreakBeforeBinaryOperators: NonAssignment
   BreakBeforeBraces: Custom
   BreakBeforeTernaryOperators: false
   BreakConstructorInitializers: AfterColon
   BreakInheritanceList: AfterColon
   BreakStringLiterals: true
   BreakTemplateDeclarations: Yes
   ColumnLimit: 100
   CommentPragmas: ''
   CompactNamespaces: true
   ConstructorInitializerIndentWidth: 4
   ContinuationIndentWidth: 4
   Cpp11BracedListStyle: false
   DerivePointerAlignment: false
   DisableFormat: false
   FixNamespaceComments: true
   ForEachMacros: [ foreach, Q_FOREACH, BOOST_FOREACH ]
   IncludeBlocks: Preserve
   IndentCaseBlocks: false
   IndentCaseLabels: true
   IndentExternBlock: AfterExternBlock
   IndentGotoLabels: true
   IndentPPDirectives: None
   IndentWidth: 4
   IndentWrappedFunctionNames: false
   InsertNewlineAtEOF: true
   InsertTrailingCommas: None
   KeepEmptyLines:
     AtEndOfFile:     false
     AtStartOfBlock:  false
     AtStartOfFile:   true
   LineEnding:      LF
   MacroBlockBegin: ''
   MacroBlockEnd:   ''
   MaxEmptyLinesToKeep: 2
   NamespaceIndentation: All
   PackConstructorInitializers: CurrentLine
   PenaltyBreakAssignment: 1
   PenaltyBreakBeforeFirstCallParameter: 1
   PenaltyBreakComment: 300
   PenaltyBreakFirstLessLess: 1000
   PenaltyBreakString: 1000
   PenaltyBreakTemplateDeclaration: 1
   PenaltyExcessCharacter: 1000000
   PenaltyReturnTypeOnItsOwnLine: 200
   PointerAlignment: Middle
   ReflowComments: true
   SortIncludes: false
   SortUsingDeclarations: false
   SpaceAfterCStyleCast: true
   SpaceAfterLogicalNot: false
   SpaceAfterTemplateKeyword: true
   SpaceBeforeAssignmentOperators: true
   SpaceBeforeCpp11BracedList: true
   SpaceBeforeCtorInitializerColon: true
   SpaceBeforeInheritanceColon: true
   SpaceBeforeParens: ControlStatements
   SpaceBeforeRangeBasedForLoopColon: true
   SpaceBeforeSquareBrackets: false
   SpacesBeforeTrailingComments: 1
   SpacesInAngles: false
   SpacesInContainerLiterals: true
   SpacesInParens: Never
   SpacesInSquareBrackets: false
   Standard: c++20
   TabWidth: 4
   UseTab: Never
   ...
   
   
   # end of file

   </details>